Do–while loop in C++

Introduction

A do-while loop lets you run a block of code at least once, then repeat it while a condition is true.

When you want to ask a user for input and repeat until they give a valid answer.

When you need to run a menu at least once and keep showing it until the user chooses to exit.

When you want to perform an action first, then check if it should continue.

When you want to repeat a task but must do it at least once no matter what.

Syntax

C++

do {
    // code to run
} while (condition);

The code inside do { } runs first, then the condition is checked.

If the condition is true, the loop repeats. If false, it stops.

Examples

This prints numbers 1 to 3 using a do-while loop.

C++

int i = 1;
do {
    std::cout << i << " ";
    i++;
} while (i <= 3);

This asks the user if they want to continue and repeats if they type 'y'.

C++

char answer;
do {
    std::cout << "Continue? (y/n): ";
    std::cin >> answer;
} while (answer == 'y');

Sample Program

This program prints "Count is: " followed by numbers 1 to 5 using a do-while loop.

C++

#include <iostream>

int main() {
    int count = 1;
    do {
        std::cout << "Count is: " << count << "\n";
        count++;
    } while (count <= 5);
    return 0;
}

Important Notes

Remember the semicolon ; after the while(condition) line is required.

Unlike a while loop, a do-while loop always runs the code block at least once.

Summary

A do-while loop runs code first, then checks the condition.

It repeats the code while the condition is true.

Use it when you want the code to run at least once before checking.

Practice

1.

What is the main difference between a do-while loop and a while loop in C++?

easy

A. do-while loop runs the code block at least once before checking the condition.

B. while loop always runs the code block at least once.

C. do-while loop checks the condition before running the code block.

D. There is no difference; both behave the same.

Solution

1. Step 1: Understand do-while loop behavior

   A do-while loop executes the code block first, then checks the condition to decide if it repeats.

2. Step 2: Compare with while loop

   A while loop checks the condition first before running the code block, so it may not run at all if the condition is false initially.

3. Final Answer:

   do-while loop runs the code block at least once before checking the condition. -> Option A

4. Quick Check:

   do-while runs once before condition check [OK]

Hint: Remember: do-while runs code first, then checks condition [OK]

Common Mistakes:

• Thinking while loop always runs code once
• Confusing order of condition check
• Assuming do-while and while behave the same

2.

Which of the following is the correct syntax for a do-while loop in C++?

?

easy

A. while (condition) do { /* code */ };

B. do ( /* code */ ) while condition;

C. do { /* code */ } while (condition);

D. do { /* code */ } until (condition);

Solution

1. Step 1: Recall correct do-while syntax

   The correct syntax uses do { ... } while (condition); with braces and a semicolon after the while condition.

2. Step 2: Identify syntax errors in other options

   do ( /* code */ ) while condition; misses braces and semicolon; C uses incorrect keyword 'until'; A reverses order incorrectly.

3. Final Answer:

   do { /* code */ } while (condition); -> Option C

4. Quick Check:

   Syntax ends with semicolon after while(condition) [OK]

Hint: Look for braces and semicolon after while(condition) [OK]

Common Mistakes:

• Omitting semicolon after while(condition)
• Using 'until' instead of 'while'
• Incorrect order of do and while keywords

3.

What will be the output of the following C++ code?

int i = 3;
do {
    std::cout << i << " ";
    i--;
} while (i > 0);

medium

A. 3 2 1

B. 3 2 1 0

C. 2 1 0

D. 1 2 3

Solution

1. Step 1: Trace the loop iterations

   Initially, i=3. The loop prints 3, then decrements i to 2. Condition i>0 is true, so repeats printing 2, then 1.

2. Step 2: Check when loop stops

   After printing 1, i becomes 0. Condition i>0 is false, loop stops. So output is "3 2 1 "

3. Final Answer:

   3 2 1 -> Option A

4. Quick Check:

   Loop runs while i>0, prints 3,2,1 [OK]

Hint: Count printed values before condition fails [OK]

Common Mistakes:

• Including 0 in output
• Starting count from 2 instead of 3
• Printing in wrong order

4.

Identify the error in this do-while loop code snippet:

int x = 5;
do {
    std::cout << x << " ";
    x++;
} while x < 10;

medium

A. The loop should end with a comma, not a semicolon.

B. The variable x should be decremented, not incremented.

C. The do keyword is misspelled.

D. Missing parentheses around the condition in while statement.

Solution

1. Step 1: Check syntax of while condition

   The condition after while must be enclosed in parentheses: while (x < 10);.

2. Step 2: Verify other parts

   Incrementing x is valid; do is spelled correctly; loop ends with semicolon, which is correct.

3. Final Answer:

   Missing parentheses around the condition in while statement. -> Option D

4. Quick Check:

   while condition needs parentheses [OK]

Hint: Always put condition in parentheses after while [OK]

Common Mistakes:

• Forgetting parentheses in while condition
• Confusing semicolon with comma at loop end
• Thinking increment is always wrong

5.

You want to write a do-while loop that asks the user to enter a positive number and repeats until the user enters a number greater than zero. Which code snippet correctly implements this?

?

hard

A. int num;
while (num <= 0) {
std::cout << "Enter positive number: ";
std::cin >> num;
}

B. int num;
do {
std::cout << "Enter positive number: ";
std::cin >> num;
} while (num <= 0);

C. int num = 0;
do {
std::cout << "Enter positive number: ";
std::cin >> num;
} while (num > 0);

D. int num;
do {
std::cout << "Enter positive number: ";
std::cin >> num;
} while (num < 0);

Solution

1. Step 1: Understand the condition to repeat

   The loop should continue while the number is less than or equal to zero, so condition is num <= 0.

2. Step 2: Check each option's condition

   int num; do { std::cout << "Enter positive number: "; std::cin >> num; } while (num <= 0); uses do-while (num <= 0); which repeats until user enters positive number. int num = 0; do { std::cout << "Enter positive number: "; std::cin >> num; } while (num > 0); repeats while number is positive (wrong). int num; while (num <= 0) { std::cout << "Enter positive number: "; std::cin >> num; } uses while loop, not do-while. int num; do { std::cout << "Enter positive number: "; std::cin >> num; } while (num < 0); repeats while number is less than zero, so zero input stops loop incorrectly.

3. Final Answer:

   int num; do { std::cout << "Enter positive number: "; std::cin >> num; } while (num <= 0); -> Option B

4. Quick Check:

   Repeat while input ≤ 0 to ensure positive input [OK]

Hint: Repeat while input is not positive (≤ 0) [OK]

Common Mistakes:

• Using wrong condition sign (> 0 instead of ≤ 0)
• Using while loop instead of do-while
• Stopping loop too early on zero input